Risks with the welding of
stainless steel (2)
Eng. N.W. Buijs
Van Leeuwen Stainless bv. Beesd
Welding of stainless steel requires the necessary knowledge in order to get
quality connections, without unexpected calamities. In Part 1 (MK 6) the
metallurgical aspects of stainless steel have already been treated in a
restricted way. This article describes the welding aspects and the possible
risks that arise when welding stainless steel in an indiscriminate way.
Since this article primarily deals
with the welding of stainless steeltypes, the metallurgy as treated in
MK 6, will not be treated again. It is
relevant, though, to mention the
different characteristic numbers of
stainless steel and also those of
unalloyed carbon steel, in order to
acquire some knowledge about the
differences (table 5).
Stainless steel is an excellent
material for welding , provided that
the right ‘rules of the game’ are
observed. Here it is also in force
that t he weakest link in the chain is
mostly the weld or also the weld
area. It is also a fact that these
weak links now don’t need to occur
anymore with the present welding
technology
and
corrosion
knowledge. It is thus often because
of ignorance and laziness, that
inferior welded joints are made.
Besides weak mechanical joints
also joints that later will become
sensitive to all sorts of corrosion
mechanisms
can
be
made.
Conversations with expert suppliers
of semi- goods (such as for
example weld additional material)
can prevent a lot of problems and
corrosion damages.
Since the physical properties of
austenite stainless steel diverge
considerably from carbon steel
during welding, this of course has
to be considered. In fact, with
stainless steel, for example, CO2 –
or autogenous welding can not be
used.
Welding of stainless steel
Since in practice quite a bit of
austenite qualities are welded, the
consideration
hereunder
will
primary relate to these stainless
steel types. Because the physical
properties of austenite stainless
steel, in contrast with the chrome
steels, diverge considerably from
those of the carbon steel, it will be
clear that these differences have a
considerable influence on the
ultimate welding process. The
linear coefficient of expansion, for
example, is 1,5 times bigger than
that of carbon steel, which means
that after a thermal strain a much
higher stress remainder is left in the
material . Fastened joints have to
be put much closer than with
carbon steel. The relatively bad
heat
conduction
capacity
of
austenite stainless steel causes
that the heat in the welded area
gets discharged much worse.
Because of this, welding in position
is
obstructed
by
the
slow
coagulation. The chance to get
inter-metallic separations on the
granule borders is increased
because of the role which is played
by the time factor. There is also the
danger that the melting bath gets
overheated. The worse conductivity
can cause that the feed electrode
gets overheated quicker. Because
of these facts it is recommended to
use the MIG-welding process of
electrode welding with thicker
sheets because of the relatively low
thermal stress. With thinner sheets
(until about 3 mm) and pipes, the
TIG-welding process is mostly
recommended. With TIG-welding
the heat is obtained from an
electrical arc between an infusible
tungsten electrode and the piece of
work. Because of this the TIGwelding process is well suited for
the welding of thin stainless steel
and pipe joints, and this in each
desired state. Also for the placing
of ground joints with thick sheets
the TIG-welding process is often
used. The welding under dust cover
is in some cases also applied.
For all the stainless steel qualities it
is important that the preparation be
optimum.
Attention
should
especially be given to the cleaning
and the pre-processing of areas
that have to be welded. The welds
should be dry, clean and especially
grease-free. Especially grease and
pencil marks contain carbon steel
which would very much like to
penetrate into the material (diffuse),
since the urge to absorb carbon
steel is so high. Grease dissociates
at increased temperatures in
carbon and hydrogen and it will be
clear that this carbon steel will
enormously corrode the weld
quality (formation of chromium
calcium carbides).
The joints
ought to be flat and undamaged
(free from burrs). The shape is
dependant on the product that has
to be welded.
As general guideline it applies to all
stainless
steel
qualities
that
additional weld materials need to
have, as much as possible, the
same chemical composition of the
parts that have to be welded.
What
is
practically
still
a
misunderstanding, is that the so
called ‘L’-qualities are specifically
meant for welding. This is not the
case, because ‘L’ stands for ‘lowcarbon’, and therefore not for
‘welding’.
It is possible to weld stainless steel
both with coated electrodes and
with
wires
(TIG/MIG-welding
process). Coated electrodes are
used on behalf of hand welding.
Advantages of hand welding are
that there is access to all the
positions and that the heat supply
is relatively low. Furthermore it
works quickly and the investments
are low. The slag also gives
protection at the back of the weld.
Disadvantages of hand welding are
that the chance on slag rests, slag
occlusions, spatters and start
colors are big, because of which
staining will always be necessary.
The process also progresses quite
slowly. Attention should be paid to
the insulating cover of the
electrodes which has to be very
dry.
With regard to the wire joints (TIG
and MIG) in general it can be
stated that the advantages are: no
slag- and splatter formation and the
weld is protected very well. With
the MIG-process the welding speed
is high and the process is easily
mechanized and automized. The
disadvantages are that both
processes are sensitive to wind
(therefore operate inside) and that
no correction of the weld bath is
possible. Care for a good backing
should also be taken.
Corrosion
Many devices of stainless steel fail
prematurely because of corrosion
damages that have arisen through
unprofessional welding. In general
it counts that the same additional
weld material must always be used
as the basic material. In practice
the corrosion damages mentioned
here below, occur mostly as a
consequence
of
incomplete
welding:
well shaped corrosion;
stress (crack) corrosion;
inter-crystalline corrosion;
weld decay;
knife-line attack;
crack corrosion.
These forms of corrosion and the
measures to prevent these as
much as possible are hereafter
further discussed.
Pit shaped corrosion
This form of corrosion is very
feared because a strong corrosion
takes place merely locally. The
stainless
steels
without
molybdenum are the most sensitive
to this form of corrosion. On
average this corrosion form shows
up mainly where halogen-ions
appear, such as fluorine-, iodine-,
bromine- and especially chlorine
ions. The cause of this corrosion
must always be searched in the
damage or interruption of the
passive layer of chromic oxide. An
extra sensitive place where this
corrosion appears, especially takes
place next to the welded join. Start
colours are the evidence of that,
because they indicate a strong
local oxidation. This oxidation
usually leads to a porous layer of
oxide which allows the electrolyte
to go through. Especially the
relatively small chlorine ion, which
is so necessary in order to keep the
object passive, often penetrates
deeper than the larger oxygen
atom. It is therefore very important
that similar start colours are
removed with staining or polishing,
possibly followed by another
passivation. This staining happens
mostly through a suitable pickle
paste, because an immersion in a
pickling bath is often not possible.
In practice, it is than often thought,
that everything is all right, but it is
forgotten that the invisible insides
can also have these start colours.
In pipe systems such kind of
problems appear regularly, even
with the use of backing-gas. The
reason is that there is still too much
oxygen in the backing-gas and/or
that the heavier backing-gas (also
called formation gas) allow the
lighter oxygen to stay above in the
pipet, because of which it still starts
local oxidation. This form of
corrosion has already prematurely
completely destroyed a lot of
channel systems. Even conduits,
through which for example, chloride
containing cooling water flows can
after one or two years be
completely full of leaks just next to
the welded joints. Effective washing
with backing-gas and the use of the
right sealing tools will offer a good
solution. Internal and external
welding
are
also
used
simultaneously on the same spot,
because of which the backing
becomes superfluous. Normally a
little hydrogen(+/-2%) is added to
the backing-gas, in order to allow
the oxygen to react with it, so that
the stainless steel is spared.
However, in practice it seems that
the effect of the hydrogen addition
often does not give the completely
desired result. Helium can also be
added to the backing-gas, in order
to chase away the oxygen at the
top of the pipe. In order to make a
good welding process the welding
chamber must first be washed with
formation gas according to the
formula in fig. 6. Empirically it is
also stated that the time of flowing
must be in such an extent that the
total volume of the backing space is
replaced about 20 times.
Since, from practice, it often
appears
that
the
complete
formation, in regards to quality, is
far from satisfactory, it can not be
sufficiently
emphasized
how
important this is. It is actually only
prudent to start with the welding
process if the amount of oxygen in
the formation gas has come under
the 50 ppm. If this critical aspect is
adequately recognized, a lot of
corrosion, like for example the well
corrosion as reproduced in figure 7,
can be prevented. It is also
possible to weld sheets by backing
them with the help of a so called
‘bearing’, out of which the
protection gas constantly flows at
the bottom of the welded joint (fig.
8). The groove width which is
necessary for this bearing is
reproduced in table 6.
Slag rests can also further increase
the risk of corrosion. Therefore
these must always be thoroughly
removed. In conclusion, it is worth
mentioning that after the application
of the first welding layer, the
backing gas has to continue flowing
when the second filling layer is
applied, because the first welding
layer will become very hot again
(big chance for oxidation). After
welding, it is also strongly
recommended carry on rinsing until
the surface has come under 250°C.
Corrosion from contamination
A variant of pit shaped corrosion is
the
so
called
contamination
corrosion, caused because strange
metal particles have ended up on
the stainless steel. Lack of oxygen
under these particles results in
activation of the surface, through
which pit corrosion will arise. The
most serious form is caused by the
contamination (infection) of carbon
steel particles, which can initiate
intense pit corrosion. Also metal
particles of all sorts of tools and
wrong wire brushes can give origin
to this form of corrosion. Therefore,
it is important that the processing of
carbon steel and stainless steel
happens separately and that the
tools are handled very consciously.
If there is still uncertainty about a
possible
contamination,
than
staining is the best solution.
Stress (crack) corrosion
Stresses often give origin to
corrosion and therefore the point is
to hold these as low as possible.
Stress areas in an electrochemical
system are always anodic and
similar spots sacrifice themselves
on behalf of the cathode, which, as
for surface, is usually many times
bigger:
this
accelerates
the
corrosion process even more. The
corrosion
phenomenon
which
arises is the trans-crystalline
corrosion, which means that the
corrosion propagates across the
metal crystals (fig. 9 and 10). This
is in contrast with the intercrystalline corrosion, where the
corrosion propagates around the
granules (fig. 11).
Small micro cracks on the surface
cause a stress rise in that specific
area, so that this spot poses itself
anodic and corrodes. During
corrosion the hydrogen ion takes
an electron from the electrolyte of
the ion metal which has come into
the solution, so that there is
formation
of
hydrogen.
This
hydrogen diffuses locally into the
surface of the stainless steel, which
leads to local brittling. This brittling
makes that the small seam
propagates easily, through which
the trans-crystalline seam (crack)
can form itself quite quickly. This
serious corrosion, in principle,
appears only with halogen-ions
(especially fluorine- and chlorine
ions).
In order to prevent stresses in
stainless steel as much as
possible, it is advised to weld the
parts as much as possible in a cold
state with the current density as low
as possible and the weld speed as
high as possible. There care should
also be taken that the welds are not
layed too closed to each other and
that the welds do not cross each
other. Attaching must therefore be
done as small as possible, so the
use of attach strips is always
dissuaded. Since, because of
shrinking, the welds also cause an
extra stress it should be looked at
that these welds have a minimum
content, without this happening at
the expense of the strength of the
connection. When welding unequal
thicknesses the chance to keep
higher remainder stresses is also
bigger and therefore, here, the task
of the constructor is to prevent it as
much as possible (fig. 12).
In practice appears that to glow
with low stress at about 1060 ˚C is
the best, this is because first the
stresses disappear and than
because possible inter-metallic
joints and other precipitates
dissolve in the matrix. There can
also arise stress corrosion if more
welded joints are put on each other
since the quality of the welds
underneath
goes
backwards
because of continually heating. It is
advisable to keep the lowest layer
on the corrosion loaded side and it
if possible to heat.
steel containing less carbon will be
much less sensitive to it than the
kinds with a higher content of
carbon. Empirically, it has been
found that the chromium carbides
arise only on the granule borders if
the carbon content is higher than
0,04 %.
In practice, it also appears that thinwalled stainless steel, generally
speaking, does not have problems
because of this phenomenon since
the cooling time is so short as a
result of the thinness of the plate
thickness.
In this last case, therefore, time is
not enough to form these carbides.
These carbides form themselves
especially in the temperature range
between 450 and 900 ˚C. At a
certain distance from the weld there
will be a so called sensitive area
which allows these carbides to
arise. Therefore, there is always
corrosion at a certain distance from
the weld and not immediately next
to the weld such as has been the
stresses the corrosion is further
intensified.
Brittling can also arise because
(Cr23 C3) the toughness of the
material is brought significantly
down on account of the hard
chromium carbides.This form of
corrosion is therefore called intercrystalline corrosion, because this
takes place between the metal
crystals (fig. 11). The part will
quickly break through because of
the present remainder stresses,
due to the notch effect which has
arisen through the corrosion. Of
course this form of corrosion can
also be obtained through other
thermal strains which last for a
certain time.
Inter-crystalline corrosion
At a short distance from the welded
joint, because of the influence of
the heat on the granule borders
and in the rule with a higher content
of carbon, chromium carbides can
arise if there is enough time for
that. More than one things appears
from the so called TTT-diagram,
which
stands
for
Time,
Transformation and Temperature
(fig. 13). The diagram shows that
these undesired carbides can form
themselves if the carbon content,
the temperature and the time lead
to it. In other words: a stainless
case of the ‘knife-line attack’.
These carbides arise on the
granule borders because there the
tendency for similar links to arise is
highest . The corrosion can even
reach such serious forms that the
granules become loose from
eachother.
These
precipitates
withdraw chromium from their
environment and through this the
environment is more base than the
rest. The environment of the
carbides through this becomes
active and will quickly corrode in a
high measure. If there are also
curve is examined on a weld, a sort
of Gauss-curve can be seen (fig.
14). If the TTT-diagram is set next
to it, it can be seen a sensitive area
arises next to the weld, which with
enough time, forms the notorious
chromium
carbides.
At
high
temperatures the solution pressure
is big enough and at lower
temperatures the diffusion speed is
too low to form the feared carbides.
This results in the so called nose
form of the TTT-curve. There
remains a sensitive area which is
rapidly affected as a consequence
of this phenomenon. In this case
Weld decay
Weld decay is actually a variant of
inter-crystalline corrosion. It is
however
specifically
named,
because
the
inter-crystalline
corrosion can also arise from other
thermal strains. If the temperature
again solutions are the application
of types with a low content of
carbon on a heat treatment after
welding. On figure 15 the corrosion
form is visible. Weld decay luckily
does not occur so much in practice
anymore, because usually enough
measures are taken against it.
Knife-line attack
Because of the relatively high
content of carbon, the types of
stainless steel which are alloyed to
titanium form titanium carbides.
These titanium carbides set
themselves on the matrix in a
disperse way, and prevent the
formation of chromium carbides.
The reason is that titanium has a
higher affinity with regard to carbon
than to chromium. Stainless steel is
in this way consolidated. However,
titanium carbides can be a cause of
pit corrosion, because these
particles can manifest themselves
as a defect on the surface. Next to
this problem and to the fact that it
cannot be polished so easily, there
is another big disadvantage and
that is that the titanium carbides will
dissociate in the heat influenced
area (disintegrate).
Titanium will than burn and the
carbon which is given off will on its
turn immediately combine with the
chromium,
with
all
the
disadvantageous
consequences.
For this reason titanium containing
additional weld material is never
applied. The consequence is that
this area has become active
because on a small strip the
chromium content has decreased
below 12%. It will be clear that
locally intense corrosion will arise
and it will have the form of a sharp
knife. This is also the reason for
which it is called knife-line attack
(fig. 16 and 17). It is not a simple
task
to
fight
against
this
phenomenon. Next to keeping the
heat-input as low as possible and
the weld-speed as high as possible,
the best remedy is still to always
use, instead of a titanium stabilized
stainless steel, a type with a low
content of carbon (L-quality). The
only disadvantage of this is that its
mechanical properties are worse.
Niobium-stabilized qualities are
obviously less sensitive to this
knife-line attack problem. Next to
additional
low
carbon
weld
materials, niobium-stabilized weld
additional materials are also used.
Crack corrosion
Even if this form corrosion as
consequence of welding does
occur very little, the constructor has
to make sure that no splits will be
present when placing the weld.
Mainly in narrow cracks, the
discharge of the medium is so
small that there will be a lack of
oxygen, which is, however, so
necessary to keep oxide hide in an
optimum form. Especially the
chlorine ion will penetrate deeply
into the narrow cracks and there
destroy the protection layer. This
last can mainly happen in
pipe/pipesheets-joints which are
welded unilaterally (fig. 18).